In a permanent magnet synchronous generator (PMSG), the excitation field is provided by permanent magnets, and the frequency of the voltages induced in the stator is directly related to the rotation rate of the rotor hub. Owing to its performance reliability, a PMSG is often the generator of choice for offshore wind energy applications. The stator can be realized as a multiple-stator machine with isolated neutral points, in which each stator winding carries a plurality of phases (usually three). A multi-stator machine may be preferred in order to achieve fault-tolerance in a demanding application such as offshore wind farm.
To convert the variable frequency of a PMSG into a constant grid frequency, a generator-side frequency converter and a grid-side frequency converter can be used in a back-to-back arrangement joined by a DC link. A frequency converter is generally realized as an arrangement of power switches such as isolated gate bipolar transistors (IGBTs) on account of their ability to switch high voltages at rapid switching frequencies. A non-switching IGBT—the result of an open-circuit fault in the power switch transistor—is one of the main causes for power converter outage in PMSG wind turbines. When a power switch fails, the corresponding generator phase currents become distorted or erratic, leading to a significant increase in torque ripple. While a slight amount of torque ripple is acceptable and indeed even unavoidable during normal fault-free or “healthy” operation of a PMSG, the significantly higher levels of torque ripple that develop after a fault cause vibrations which may in fact cause damage to the generator or to other components.
One of the advantages of a multi-phase PMSG wind turbine generator is its ability to carry on operating even with one or more open-circuit faults in its generator-side converter. In the case of multi-stator PMSG with multiple isolated neutral points, the appropriate winding (i.e. all phases) is generally taken out of service. However, this results in a significant decrease in power output, with a corresponding loss in revenue.